http://angrybirds.wikia.com/wiki/Mauro#Mauro

Caged Birds are a few species of birds (purple, pink, red, blue, yellow and green; also a Blue and Gold Macaw and a Scarlet Ibis) that appear in Smuggler's Den, Airfield Chase and Smuggler's Plane as prisoners of the Smugglers. It is implied that they, like the Angry Birds (Red, Blue, Yellow, Orange, Green, White, Black and Big Brother.) are rare species as the official game trailer refers to them as, "exceptionally rare species". The other birds try to free them from their cages by slinging themselves at the cages; when a cage breaks, the caged bird flies away.